The present invention relates generally to the field of magnetic resonance imaging (MRI). More particularly, the present invention relates to the field of MRI chemical-shift excitation.
In a typical magnetic resonance imaging MRI) system, a subject such as a human body is placed in a static magnetic field such that selected nuclear magnetic dipoles of the subject preferentially align with the magnetic field. The MRI system then applies radio frequency (RF) pulsed magnetic fields to cause magnetic resonance of the preferentially aligned dipoles and detects RF magnetic resonance (MR) signals from the resonating dipoles for reconstruction into an image representation. The MRI system typically scans the region to be imaged by applying RF pulse sequences to the subject while imposing time-varying magnetic field gradients with the static magnetic field.
In imaging most tissues with MRI, the hydrogen protons from water are preferably detected as most soft tissues are composed of greater than approximately eighty percent water. Unfortunately, fat is also largely composed of hydrogen protons and may therefore appear as an unwanted or unnecessary component in many hydrogen MR images. A variety of methods have been developed to help eliminate the effect of fat magnetization from hydrogen MR images and thereby improve the contrast between normal and pathologic tissue in a variety of anatomic locations such as, for example, the liver and pancreas, the orbits, the breast, bone marrow, and the coronary arteries. Water excitation methods apply an RF pulse sequence to tip water magnetization and not fat magnetization for detection. Fat suppression methods apply an RF pulse sequence to tip fat magnetization and not water magnetization, eliminate the fat magnetization, and then excite the water magnetization for detection. Such methods are able to tip water and fat magnetization in a selective manner because of the chemical shift difference in resonant frequency between water protons and protons in the methylene (xe2x80x94CH2) groups of fat molecules.
The chemical shift difference between two chemical species in which excitation of one and elimination of the other is desired is given by xcex4 in parts per million (ppm). For water and fat protons, the chemical shift difference is approximately 3.5 ppm in accordance with the following equations:                               ω          water                =                              γ            ⁢                          xe2x80x83                        ⁢                          B              0                                =                      xe2x80x83                    ⁢                                    ~              2                        ⁢                          π              ⁡                              (                                  64.05                  ⁢                                      xe2x80x83                                    ⁢                  megaHertz                  ⁢                                      xe2x80x83                                    ⁢                                      (                    MHz                    )                                                  )                                                                                  xe2x80x83                ⁢                              at            ⁢                          xe2x80x83                        ⁢                          B              0                                =                                    ~              1.5                        ⁢                          xe2x80x83                        ⁢            Tesla            ⁢                          xe2x80x83                        ⁢            or                                                  =                  xe2x80x83                ⁢                              ~            2                    ⁢                      π            ⁡                          (                              8.5                ⁢                                  xe2x80x83                                ⁢                MHz                            )                                                                    xe2x80x83                ⁢                              at            ⁢                          xe2x80x83                        ⁢                          B              0                                =                                    ~              0.2                        ⁢                          xe2x80x83                        ⁢            Tesla                                                            Δ          ⁢                      xe2x80x83                    ⁢                      ω                          water              ⁢                              -                            ⁢              fat                                      =                  xe2x80x83                ⁢                                            ~              2                        ⁢                          π              ⁡                              (                                  64.05                  ⁢                                      xe2x80x83                                    ⁢                  MHz                                )                                      ⁢            δ                    ⁢                      xe2x80x83                    =                                    ~              2                        ⁢                          π              ⁡                              (                                  224                  ⁢                                      xe2x80x83                                    ⁢                  Hz                                )                                                                                  xe2x80x83                ⁢                              at            ⁢                          xe2x80x83                        ⁢                          B              0                                =                                    ~              1.5                        ⁢                          xe2x80x83                        ⁢            Tesla            ⁢                          xe2x80x83                        ⁢            or                                                  =                  xe2x80x83                ⁢                                            ~              2                        ⁢                          π              ⁡                              (                                  8.5                  ⁢                                      xe2x80x83                                    ⁢                  MHz                                )                                      ⁢            δ                    ⁢                      xe2x80x83                    =                                    ~              2                        ⁢                          π              ⁡                              (                                  29.75                  ⁢                                      xe2x80x83                                    ⁢                  Hz                                )                                                                                  xe2x80x83                ⁢                              at            ⁢                          xe2x80x83                        ⁢                          B              0                                =                                    ~              0.2                        ⁢                          xe2x80x83                        ⁢            Tesla                              
where xcfx89 is the Larmor frequency of the nuclei of interest, xcex3 is the gyromagnetic ratio of the nuclei of interest, and B0 is the applied static magnetic field.
One common fat suppression method applies binomial sets of RF pulses at specific amplitudes and specific interpulse intervals to tip fat magnetization into the transverse or detection plane while restoring water magnetization to the longitudinal axis. The amplitudes of the RF pulses are set such that their sum is approximately zero when observed by a water molecule (i.e., on resonance), and the duration of each interpulse interval is set, for example, to xcfx80/xcex94xcfx89=xcx9c1/(448 Hz) at B0=1.5 Tesla=xcx9c2.2 milliseconds (ms) such that the water and fat protons precess by approximately 180xc2x0 or xcfx80 radians with respect to one another. Once in the detection plane, the fat magnetization may be spoiled or destroyed. A selective RF pulse may then be applied to tip the remaining longitudinal magnetization into the detection plane. As the time interval between the tipping of fat magnetization into the detection plane and spoiling is relatively short, the remaining longitudinal magnetization tipped by the selective RF pulse is substantially all water magnetization. Exemplary prior art binomial RF pulse sequences include 1-(-1), 1-(-2)-1, and 1-(-3)-3-(-1) sequences.
The application of a prior art binomial 1-(-1) RF pulse sequence for fat suppression is illustrated in graph form in FIGS. 1A, 1B, 1C, 1D, and 1E. As illustrated in FIG. 1A, water magnetization 11 and fat magnetization 12 are initially aligned with the static magnetic field B0 along the z-axis at equilibrium. A first RF pulse in the 1-(-1) sequence tips both water magnetization 11 and fat magnetization 12 by approximately 45xc2x0 as illustrated in FIG. 1B. During an interpulse interval of approximately 2.2 ms for B0=xcx9c1.5 Tesla, fat magnetization 12 precesses by rotating approximately 180xc2x0 about the z-axis such that water magnetization 11 and fat magnetization 12 are approximately 180xc2x0 out of phase as illustrated in FIG. 1C. A second RF pulse in the 1-(-1) sequence tips both water magnetization 11 and fat magnetization 12 by approximately xe2x88x9245xc2x0, restoring water magnetization 11 to the z-axis while tipping fat magnetization 12 into the detection plane as illustrated in FIG. 1D. Fat magnetization 12 is then spoiled by a magnetic field gradient pulse as illustrated in FIG. 1E, and water magnetization 11 may then be tipped from the z-axis into the detection plane by a selective RF pulse.
Adding more RF pulses in a binomial sequence helps improve the spectral width of the saturation in an inhomogeneous magnetic field. At B0=xcx9c1.5 Tesla, a binomial 1-3-3-1 RF pulse sequence, for example, may be used for fat suppression.
Applying binomial sets of RF pulses in relatively lower magnetic fields, however, incurs relatively longer repetition times TR and therefore scan times as the duration of each interpulse interval is inversely proportional to the strength of the magnetic field B0. At B0=xcx9c0.2 Tesla, for example, a binomial RF pulse sequence requires an approximately 16.8 ms interpulse interval as compared to the approximately 2.2 ms interpulse interval required at B0=xcx9c1.5 Tesla. For longer pulse sequences that are required for adequate suppression in an inhomogeneous magnetic field, the time penalty incurred is too great for many imaging applications. A binomial 1-3-3-1 RF pulse sequence, for example, requires approximately 50 ms in total interpulse interval time at B0=xcx9c0.2 Tesla.
Also, the effectiveness of binomial RF pulse sequences in suppressing fat may be compromised in relatively lower magnetic fields as the relatively longer interpulse intervals together with the reduced relaxation time T1 for fat in the lower magnetic field allow significant fat magnetization regrowth. Relatively longer interpulse intervals also allow greater water magnetization decay as determined by the relaxation time T2 for water.
Another common fat suppression method relies upon the regrowth of fat magnetization. Fat and water magnetization regrow at different rates as determined by their respective relaxation times T1. Following application of an inverting RF pulse, regrown magnetization will effectively cancel the inverted magnetization after a certain time period TI=ln(2)*T1=xcx9c0.693*T1.
The application of a prior art inversion recovery RF pulse sequence for fat suppression is illustrated in graph form in FIGS. 2A, 2B, and 2C. As illustrated in FIG. 2A, water magnetization 21 and fat magnetization 22 are initially aligned with the static magnetic field B0 along the z-axis at equilibrium. An inverting RF pulse tips both water magnetization 21 and fat magnetization 22 by approximately 180xc2x0 as illustrated in FIG. 2B. After TI=xcx9c160 ms at B0=xcx9c1.5 Tesla or TI=xcx9c110 ms at B0=xcx9c0.2 Tesla, fat magnetization regrowth 23 effectively cancels inverted fat magnetization 22 as illustrated in FIG. 2C. Water magnetization 21 may then be tipped into the detection plane by a selective RF pulse.
At TI=xcx9c110 ms or xcx9c160 ms, the time required for fat magnetization regrowth incurs relatively longer repetition times TR and therefore scan times. Inversion methods for fat suppression may also suppress the magnetization from tissues having a relaxation time T1 comparable to that of fat and alter the contrast between tissues from that normally achieved independently from the spin-echo or gradient-echo portion of the scan.
In accordance with the present invention, a method determines a radio frequency (RF) pulse sequence of N RE pulses and Nxe2x88x921 interpulse interval(s) for use in magnetic resonance imaging (MRI) of at least a portion of a subject comprising two chemical species, such as water and fat for example, having a chemical shift difference in resonant frequency. The number N of RF pulses is an integer greater than one and may be greater than or equal to three, for example.
For the method, a numerical optimization is performed to determine an amplitude and phase angle for each of the N RF pulses and a duration for each of the Nxe2x88x921 interpulse interval(s) so as to excite magnetization of a selective one of the two chemical species for MRI detection upon application of the RF pulse sequence to at least a portion of the subject.
A total duration of the RF pulse sequence may be constrained in performing the optimization. The optimization may be performed so as to help minimize magnetization of the selective one of the two chemical species along a predetermined axis and help maximize magnetization of the other one of the two chemical species along the predetermined axis upon application of the RF pulse sequence to at least a portion of the subject while constraining the total duration of the RF pulse sequence.
The optimization may also be performed so as to help minimize a total duration of the RF pulse sequence while constraining magnetization excitation of the selective one of the two chemical species for MRI detection upon application of the RF pulse sequence to at least a portion of the subject. The optimization may be performed so as to help minimize total interpulse interval time while constraining magnetization excitation of the selective one of the two chemical species for MRI detection upon application of the RF pulse sequence to at least a portion of the subject. Magnetization of each of the two chemical species along a predetermined axis may be constrained in performing the optimization.
The optimization may further constrain total interpulse interval time, the duration of each interpulse interval, the phase angle for each RF pulse, and/or a magnetization tip angle to be effectuated by each RF pulse upon application to at least a portion of the subject.
The determined RF pulse sequence may be applied to at least a portion of the subject, and an image may be reconstructed from resulting RF magnetic resonance (MR) signals detected from at least a portion of the subject.
Also in accordance with the present invention, a magnetic resonance imaging (MRI) system comprises a static magnet, pulse sequence apparatus, pulse sequence control apparatus, and a computer system.
The static magnet produces a magnetic field along a predetermined axis relative to at least a portion of the subject in an examination region. The magnetic field may be approximately 0.2 Tesla, for example. The pulse sequence apparatus creates magnetic field gradients in the examination region, applies radio frequency (RF) pulsed magnetic fields in the examination region, and receives RF magnetic resonance (MR) signals from the examination region. The pulse sequence control apparatus controls the pulse sequence apparatus to apply an RF pulse sequence in the examination region so as to excite magnetization of a selective one of the two chemical species. The RF pulse sequence is determined in accordance with the numerical optimization which may be performed by the computer system. The computer system reconstructs an image from received RF MR signals resulting from application of the RF pulse sequence in the examination region.